Capacitor banks tuned by calibration circuitry can be applied for process-voltage-temperature (PVT) compensation in passive mixers. FIG. 1 shows the circuit of a double-balanced current-mode passive mixer 100, loaded with tunable capacitor banks C1, C2, . . . , CN. First, the input voltage Vin is converted to current Iin by a transconductance amplifier Gm. Then the current Iin is fed into the switches controlled by complimentary local oscillator (LO) signals, and is accumulated on the load capacitor banks C1, C2, . . . , CN.
The local oscillator (LO) generates complimentary signals which are beat against the signal of interest to mix it to a different frequency. The local oscillator (LO) signals control the switches in mixer 100, while the signal of interest is injected at the input of the mixer 100, to produce the sum and difference of their frequencies. These are the beat frequencies. Normally for a down converter, the beat frequency is the difference between the two; while for an up converter, the beat frequency is the sum of the two.
The current accumulated on the capacitor banks C1, C2, . . . , CN provides the output voltage Vout. The capacitor banks C1, C2, . . . , CN are controlled by a calibration circuit 102, providing a total capacitance of:
                              C          total                =                              ∑                          k              =              0                        N                    ⁢                                    h              k                        ⁢                          C              k                                                          (        1        )            
where hk=0 or 1, representing the digital control bits; and Ck=C1, C2, . . . , CN.
The conversion gain CG of the mixer can be derived as:
                    CG        =                                            v              out                                      v                              i                ⁢                                                                  ⁢                n                                              =                                    2              π                        ·                                          G                m                                            2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  f                  out                                ⁢                                  C                  total                                                                                        (        2        )            
where fout is the output frequency; and Gm is the transconductance of a transconductance amplifier, commonly implemented by active devices, as shown in FIG. 2. IB is the DC biasing current. The transconductance Gm is given by:
                              G          m                =                                            i              o                                      v                              i                ⁢                                                                  ⁢                n                                              =                                    g              m                        2                                              (        3        )            
where gm is the small-signal transconductance of the transistors M1 and M2. A transconductance amplifier (gm amplifier) puts out a current proportional to its input voltage. In network analysis the transconductance amplifier is defined as a voltage controlled current source (VCCS). In field effect transistors, transconductance is the change in the drain/source current divided by the change in the gate/drain voltage with a constant drain/source voltage. Typical values of gm for a small-signal field effect transistor are also 1 to 10 millisiemens.
Substituting (3) into (2), provides the mixer conversion gain as:
                    CG        =                                            v              out                                      v                              i                ⁢                                                                  ⁢                n                                              =                                    1              π                        ·                                          g                m                                            2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  f                  out                                ⁢                                  C                  total                                                                                        (        4        )            
In sub-micron processes, the small-signal transconductance gm can vary in magnitude by 2-3 times due to PVT variation. Thus, to compensate for the PVT-induced gain variation, the load capacitance Ctotal covers a wide range, resulting in a large silicon area, especially when a low density capacitor (such as a metal capacitor) is used (for example, to achieve good linearity of the mixer).
Further, the calibration circuit 102 was needed to detect the mixer gain variation, and then to tune the capacitor banks, resulting in significant power consumption.
The PVT compensation circuitry and calibration circuitry 102 consume high power and a large silicon area, especially when the PVT-induced gain variation is large, as in the case of current-mode passive mixers 100.